10/10/2012 - PLEASE NOTE. At least as of SeaMonkey 2.12.1 (maybe earlier) the pages in sections 4 and 5 below no longer work.
There is the Password Exporter extension that
allows exporting to a .csv file which can be opened in Excel. As of 10/10/12 it has not been updated to work on SM 2.12.1. I edited the
install.rdf file to allow this. You can get it here.

Password Viewing

SeaMonkey (and Mozilla Suite (1.7.12 release) by default stores passwords in Base 64
encoding unless you activate Master Password in which case they are
stored encrypted. As of Firefox .7 the default is encrypted.
Both browsers now have a "view passwords" function in the Manage Passwords dialogue so
this info is kind of academic at this point.

Base 64 passwords are stored in a plain text file in your profile
directory. The usernames and passwords in this file can be fairly
easily encoded and decoded.

The file historically was named in the format: [8 random numbers].s (for example, 56712987.s).
Firefox now uses signons.txt.

Here are some different ways to manipulate, view, and edit Base 64 passwords.

2. You can use the location bar (URL entry bar) in the Mozilla browser to invoke a javascript command (thanks to Karsten).
The following javascript commands can be copied and pasted into the location bar in your browser

javascript:btoa("mypassword")

will result in "bXlwYXNzd29yZA=="

And

javascript:atob("bXlwYXNzd29yZA==")

will decode it back to "mypassword."

3. You can store bookmarklets (a bookmark containing a javascript) to encode and decode passwords in Base
64. Once you have the bookmarklets saved, you can simply open your
password file in a browser window, highlight a username or password,
click the deCode bookmarklet, and the decoded word will show in the
browser window. Click BACK to go back to your file and do it again.

An encoding bookmarklet allows you to highlight a word in your browser
and encode it. You can then highlight, copy, and paste that into your
password file using Notepad or another text editor.

4. Download
this page to your hard drive and load it into Firefox or Mozilla (right-click the link to save). It will display your
passwords in the browser. This will not work online due to browser security!
Thanks to "ernie" at netscape.mozilla.firefox who "borrowed" the code from the Firefox 1.0.7
source tree.

4b. Here's a newer version that neatens things up a bit.
Thanks to Andrew Poth.

After using either of the two files above you can save the resulting file and it will include all of your password information.
Use "File - Save Page as" or right-click in the page and choose "Save Page as." Name the file and select where to save it.
It will contain all your password info as well as the original script. To prevent having the script run every time you
view the saved file in your browser, open the file in a text editor and delete the comments and script portions. Save the result.

MORE INFO

In the result file will be your password info. But the saved file will also contain all the original javascript code too.
In order to prevent the security challenge every time you open the html file you can delete the javascript.
The scripts are denoted by:

<script type="text/javascript">
all the stuff here is the script
...
...
</script>

Delete all of the script, save the file. Open it and you won't have to go through the security check/pause.
Obviously the file contains only those passwords in effect when the file was originally saved.

5. (August 5, 2008) In Firefox 3 the security routine has apparently changed and the
HTML/javascript solutions in section 4 above no longer work. Here is
a new version that is supposed to work on FF3. I have only tested it
briefly and it seems to work fine. And here is a link to
a bit more info.

And, finally, I added some minor "prettying" and formatting to tailor it to my liking to produce this version. Again, it must be downloaded and run from your local disk, not the Internet!, for it to work.